The present invention relates to position measuring systems of the type comprising at least one illuminating arrangement, a scanning unit, a graduation carrier which defines at least one graduation field, at least one photodetector for generating a signal in response to light from the illuminating arrangement modulated by the scanning plate and the graduation field, and an evaluating circuit for evaluating the signal.
Such measuring systems are commonly used in incremental length or angle measuring systems. Typically, a system is provided for reproducing a measuring system zero point in a reliable manner.
German DE-OS 18 14 785 describes the structure of a reference mark and a system for generating a reference pulse in response to this reference mark.
A sufficiently exact reference pulse can be derived from such a reference mark however, only if the scanning distance is maintained at a small value. When small scanning distances are used, the scanning distance is subject to correspondingly close tolerances with respect to scanning distance fluctuations.
The photo-electric scanning of such prior art reference marks generates one beat (Eintakt) signals. In order to obtain the push-pull or pseudo-push-pull signals required for reliable evaluation it is common practice to provide two reference marks or a reference mark and a field (such as a mirror) to generate a single reference signal. In the event the reference marks are fouled to differing degrees or in the event of distance variations in the scanning of the separate reference marks, the generated photo-electric signal can change in such a way as to prevent reliable evaluation of the reference signal.
Furthermore, prior art reference marks of the type described above do not include encoding within the graduation field in order to make one reference mark distinguishable from another.
German DE-OS 23 16 248 describes a photo-electric position marking system that operates with phase grids, which allow a larger scanning distance between the two grids which are shiftable with respect to one another. Corresponding reductions in sensitivity to scanning distance variations are thereby obtained. This document however provides no suggestion as to how a reference mark can be scanned and evaluated in a sufficiently reliable manner.